Most commercial polyethylene is produced using Ziegler-Natta catalysts, but polymers made from single-site catalysts, including metallocenes, are gaining market acceptance. While metallocene-based polyethylene resins (m-PE) can provide films and other articles with superior physical properties, they can lack the processability advantages of Ziegler resins. The processability of m-PE can be enhanced by introducing long-chain branching, but so far, only limited progress has been achieved. In contrast, low density polyethylene (LDPE), which is used for extrusion coatings, sheets, blow-molded articles, and some films is highly branched. Unfortunately, this amount of branching is too much for many film applications. Moreover, LDPE is produced using a high-pressure process rather than the solution, slurry, or gas-phase phase processes that now dominate the industry for making linear low density polyethylenes (LLDPE).
Single-site catalysts based on transition metal indenoindolyl complexes are known (see, e.g., U.S. Pat. Nos. 6,232,260 and 6,451,724). Indenoindolyl complexes are versatile because a wide variety of indanone and arylhydrazine precursors can be used to produce the ligand precursors. Thus, substituent effects can be exploited and catalyst structure can be altered to produce improved polyolefins. However, exploiting the versatility of indenoindolyl complexes demands an appreciation of the interrelationship among resin properties.
We have described an analytical approach that bridges the gap between gel permeation chromatography (GPC) and rheology-based information (see W. Yau and D. Gillespie, Polymer 42 (2001) 8947; W. Yau, TAPPI 2005 PLACE Conference Proceedings, TAPPI Press, Atlanta, Session 19, Paper 19-1; and C. Enos, K. Rufener, J. Merrick-Mack, and W. Yau, Waters International GPC Symposium Proceedings, Jun. 6-12, 2003, Baltimore, Md.) In particular, we utilized a combination of 3D-GPC and 3D-TREF (temperature rising elution fractionation) techniques that use on-line light-scattering, intrinsic viscosity, and concentration (refractometer or infrared) detectors. The techniques provide detailed information about polymer microstructure and enable detection of subtle differences in polymer molecular weight, molecular weight distribution, short-chain branching, and long-chain branching. As we explained, long-chain-branching index (LCBI) values obtained from rheology can be correlated with a GPC-based measure of long-chain branching called gpcBR. Particularly when viewed with information such as differential scanning calorimetry (DSC) melting points, density, and melt indices, the 3D-GPC and 3D-TREF techniques are valuable tools for characterizing polyolefin resins.
Identifying resin characteristics that translate into improved films, coatings, sheets, and molded articles is a continuing challenge. It would be valuable, for example, to find resins that provide good impact resistance and high heat-seal strength over a wide temperature range. Ideally, even high-molecular-weight resins could be extruded at low pressures to allow for increased film production rates. The industry would benefit from unique resins that combine attributes of essentially linear polyethylenes (e.g., m-LLDPE) and more highly branched ones (e.g., LDPE) with the processability advantages of Ziegler-Natta resins.